The Queen turns 90 this month - a global icon in tweed suits and sensible shoes.

In her 10th decade, she is a doughty survivor who still rides her horses regularly, despite her grand age. When her birthday arrives, on April 21, there will be a programme of pageants to celebrate her milestone.

But this year marks another anniversary for Elizabeth - one that will not be part of the official Royal calendar or the subject of cosy documentaries and interviews with Kate and Wills.

This autumn it is 80 years since the abdication crisis. When the future queen was just 10, her uncle, King Edward, scandalised the world by giving up his throne to marry his mistress, the twice-divorced Wallis Simpson, sending a shock wave through the Royal family.

One journalist dubbed it "the greatest story since the resurrection". It was an event on which Elizabeth's destiny would hinge. Without it, she would never have become Queen.

As a child, Elizabeth had not been raised to reign. Her father, George (known as Bertie), the shy, stammering youngest son of King George V, was forced to take over the throne when his brother chose his lover over his country. Edward would be forever known to history as the black sheep and Bertie as the stalwart who stepped in.

To some it is one of history's great romances, a clash between old-fashioned Royal values and the emerging Hollywood-espoused principle of following one's heart at all costs.

In this romantic version, Edward is the handsome playboy King, Wallis his glamorous soulmate, their love powerful enough to compromise the monarchy. At the time, however, it was seen as a travesty - the theft of their leader at a crucial moment in history, as Britain faced into a second World War, by a scheming American woman who would stop at nothing to become Queen.

Wallis would go down in history as the woman who nearly brought down the monarchy, but Elizabeth, it looks likely, will go down as the woman who overcame adversity and restored stability.

She has weathered no shortage of controversies, challenges and scandals herself, but under her leadership the Royal family has reformed as a slick, successful PR machine.

Just 25 when she solemnly undertook her vows to serve her country, she was still a girl - though a beautiful one with the figure of a 1950's pin-up. At her side was Prince Philip, the handsome naval cadet she had married several years earlier. She had met Philip when she was just 13, and was so taken with the 18-year-old at the time that she reportedly told her friends he "looked like a Viking god".

He had a reputation as a ladies' man and the King's private secretary made his disapproval known, saying that Philip was "rough, ill-mannered, uneducated and would probably not be faithful". But Philip has remained steadfast in his devotion.

There was a challenging adjustment period after she acceded to the throne and he, as her husband, was obliged to sacrifice his naval job which gave him so much satisfaction

Initially, his role in the Royal family grated, according to a senior cleric who knows the Royal family well, who says Philip became "an increasingly irritable presence in the palace as he sought to find his feet in a court that accorded him no status and attempted to bypass him at every stage".

But the Queen had learned the lesson of endurance under duress well from her father, who despite the frailties of his character, stood firm as the storm surrounding the abdication crisis raged. It has been her habit, whenever she has faced difficulties in her personal life, her public life and the press, to hold firm and to respond with reason and calm rather than high emotion.

Sometimes, this has been to her detriment. When Princess Diana was killed at the age of 36 in a road accident in Paris, the nation went into mourning. As the public turned up in their droves at Buckingham Palace, the Queen herself remained in Balmoral with her grandsons William and Harry, eschewing the opportunity to share in the tidal wave of grief which poured onto the palace steps.

Then, when she decided not to fly the flag at half mast, the public became furious at what they saw as her cold and remote demeanour. One editorial in The Sun fumed that: "There has been no expression of sorrow from the Queen on behalf of the nation. Not one word has come from a Royal lip, not one tear has been shed in public from a Royal eye."

By the time she died, Diana had become a tricky problem. The Queen's reserve in the aftermath of the accident provided space for hysterical speculation and conspiracy theories to flourish. Most of these have been widely rubbished, but some of the mud stuck.

In her book about Elizabeth, Royal biographer Ingrid Seward claimed that her response at hearing of the accident was slightly off-colour. "At first it was thought that, though the car crash in the Pont de l'Alma tunnel was serious, Diana had not been killed," Seward writes. "According to one witness present, when Elizabeth heard the initial news, she mused out loud: 'Someone must have greased the brakes.'"

Yet, according to Seward, despite the turmoil they ultimately caused each other, there had been deep and sincere affection between HRH and Lady Di at an earlier stage. "She is one of us," the Queen once wrote to a friend before Diana and Charles were married. "I am very fond of all three of the Spencer girls."

As Diana's profile grew, Seward claims that the Queen, recognising the princess's emotional fragility, made attempts to protect her from press interference.

But as Diana's relationship with Charles turned increasingly sour, so did her relationship with Elizabeth. By the 1990s it had become increasingly clear that the "fairy tale" marriage of Prince Charles and Diana was falling apart. A volley of acrimony and resentment was reported in the press.

Things came to a head in 1992 - the year which would go down in history as the Queen's annus horribilis. In May, Andrew Morton published Diana, Her True Story, an account which blew open the private goings-on behind palace doors.

With Diana's permission, the book exposed Charles's long-standing relationship with Camilla Parker Bowles, the Princess's bulimia and her attempts at suicide. Then, in August, came 'Squidgygate' - the publication in The Sun of intimate phone-calls between Diana and her lover James Gilbey. The Queen, who was reportedly furious about the debacle, and its impact on the public image of the Royal family, ordered an inquiry.

But worse was in store. In November of that year, it was Charles's turn. The Daily Mirror had taken possession of illegally recorded conversations between the Prince and his long-standing lover Camilla Parker Bowles. The details were mortifying.

It was, according to Morton, an exposure which "persuaded most of the nation that the heir to the throne, the would-be Defender of the Faith, was an adulterer".

For the Queen, for whom preserving the dignity and decorum of her office has always been paramount, this was a terrible blow.

The so-called 'War of the Wales's' was at its height and it played out in messy, visceral detail in the world's press. From every side, the institution she desired to protect was getting a drubbing in the papers. And the bonds of marriage within her family were falling apart. It wasn't just Charles and Diana.

Princess Anne's marriage to Mark Phillips officially ended in April 1992, and the Queen's other son, Prince Andrew, was having a hard time of his own. In August of the same year, it was Sarah Ferguson who was in trouble.

Pictures were published of Andrew's estranged wife on holidays in the south of France in a compromising position with her so-called "financial advisor", a Texan by the name of John Bryan, in which he was seen to be sucking her toes.

Of her four children, it is Prince Edward, often described as her favourite, who has had an ostensibly happy (first) marriage.

No wonder, then, that when the Queen came to give her traditional speech that year, she spoke plainly: "1992 is not a year I shall look back on with undiluted pleasure. It has turned out to be an annus horribilis."

But the storm was far from over and, in 1995, Diana went further still, giving the infamous interview with Martin Bashir in which she declared: "Well, there were three of us in this marriage, so it was a bit crowded."

Later that year, in an attempt to resolve the mess, the Queen took an unprecedented step, and wrote to both Charles and Diana, urging them to "seek an early divorce".

Diana's death brought this long and troubled period of royal history to a close. But it was clear, even to a traditionalist like Elizabeth, that the institution had been damaged. It needed to modernise and change.

In a gesture that recalled her grandmother Mary, she and Charles both stripped down their civil lists and announced that they would pay taxes - recognition of the fact that the survival of 'The Firm' depended on public goodwill more than anything else.

Though initially circumspect about Charles's ongoing relationships with Camilla, Elizabeth eventually had a change of heart which demonstrated her flexibility of mind, and endeared her to the public further.

When Camilla and Charles married, she gave a warm and eloquent speech at their reception. Using a racing analogy, she spoke of the "terrible obstacles" they had overcome. "They have come through and I am very proud and wish them well. My son is home and dry with the woman he loves. Welcome to the winners' enclosure," she said.

But it was the wedding of William to Kate Middleton that seemed to confirm that a new era and a new image had been achieved.

One courtier recalled the event by saying: "The Queen was so happy on the wedding day, she was practically skipping. Seeing her family full of joy, but also seeing the public support and excitement touched her greatly."

So, plenty for Her Majesty to celebrate when she blows out 90 candles on April 21, then.